A high definition television (HDTV) signal is generally understood to contain image information with approximately twice the horizontal and vertical resolution of a standard television image (e.g., NTSC), and with a wider aspect ratio compared to a standard television image. An HDTV signal may be broadcast using a simulcast technique, where two versions of the same program material are broadcast simultaneously via separate standard 6 MHz channels. One of the two program versions contains standard definition NTSC information broadcast on one channel, while the other contains high definition information broadcast on the other 6 MHz channel.
The high definition simulcast channel can be implemented (in a standard 6 MHz channel) by using digital signal processing including signal coding and data compression techniques. In the process of coding digital high definition video data for transmission, the video data may be compressed and transmitted in a layered coded format. The layered format may incorporate header data identifying data segments such that, upon loss of data during transmission, a receiver can find an appropriate re-entry point in the received data stream. Extra protection against lost or corrupted data causing confusion or disrupted service at the receiver can be provided by arranging coded video data in transport blocks. The transport blocks include additional header data identifying relatively small portions of the video data. The header data advantageously may include pointers which indicate data re-entry points within respective transport blocks.
The video data may be compressed in a variety of ways, such as in MPEG format or a similar format. MPEG is a standardized coding format being established by the International Organization for Standardization. The standard is described in the document "International Organization for Standardization", ISO/IEC DIS 11172, Coding of Moving Pictures and Associated Audio for Digital Storage Media, Rev. Nov. 23, 1991, which document is incorporated herein by reference for description of the general code format. A system which processes a high definition television signal advantageously using MPEG-like processing and video data transport blocks with associated headers is described in copending U.S. patent application "Apparatus for Segmenting Encoded Video Signal for Transmission", A.A. Acampora et al., Ser. No. 814,207. In that system, a transport processor is used to form data words into transport data packets which comprise a transport block. The transport processor also generates the required transport headers, and merges the headers with the appropriate transport data packets to form transport blocks.
An exemplary HDTV signal processing system which may be used advantageously in a simulcast context is described in copending U.S. patent application Ser. No. 650,329 of H. E. White, filed Feb. 4, 1991. In that system a television signal including high definition image information is transmitted using two 32-QAM (Quadrature Amplitude Modulated) carriers frequency multiplexed in a 6 MHz transmission band. One of the carriers conveys high priority information, while the other carrier conveys (relatively lower) standard priority information. The high priority information is the information needed to create a viewable image, albeit less than a perfect image, and is conveyed with significantly more power than the standard priority information, which is the remaining information.
As discussed in the Acampora et al. patent application, a video signal of the dual carrier, dual priority type described in the White patent application is initially compressed in conformance with an MPEG-like format. Thereafter, MPEG-type signal codewords are parsed into two bitstreams in accordance with the relative importance of the respective codeword types. The bitstreams of relatively greater and lesser importance are assigned high priority and standard (low) priority status respectively, and are conveyed by the respective carriers.
It is herein recognized as desirable to parse the MPEG-type codewords into a high priority bitstream and a standard priority bitstream with a minimum of actual or likely disruption of the signal processing operation. In particular, it is recognized as desirable to accomplish such parsing without having to interrupt the bitstream such as by interrupting an associated system clock, thereby eliminating clock stop/start synchronization difficulties.